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graphene composite during cycling as the anode material for lithium-ion batteries
Nano Energy (2014) 10, 172–180
Available online at www.sciencedirect.com
journal homepage: www.elsevier.com/locate/nanoenergy
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Enhanced electrochemical performance of MnO nanowire/graphene composite during cycling as the anode material for lithium-ion batteries Su Zhang1, Lingxiang Zhu1, Huaihe Songn, Xiaohong Chen, Jisheng Zhou State Key Laboratory of Chemical Resource Engineering, Beijing Key Laboratory of Electrochemical Process and Technology for Materials, Beijing University of Chemical Technology, Beijing 100029, PR China Received 27 June 2014; received in revised form 7 September 2014; accepted 11 September 2014 Available online 22 September 2014
KEYWORDS
Abstract
Graphene; MnO nanowire; Lithium ion batteries; Capacity enhancement; Interphase interaction
Generally, the capacity of lithium-ion batteries (LIBs) will fade gradually during cycling especially under large current densities because of the structural collapse of electrode materials. Herein, dramatic and favorable electrochemical performance enhancement was observed in a simply achieved MnO nanowire/graphene composite during long-term cycling when utilized as the anode material for LIBs. Characterized by high-resolution transmission electron microscopy, X-ray diffraction and Fourier-transfer infrared spectroscopy, the MnO nanowires were gradually collapsed to nano-spindles and further nanoparticles but these newly formed nanoparticles are still stably anchored on the graphene lamellas. Correspondingly, the specific capacity of the MnO nanowire/graphene electrode exhibited a significant enhancement with a durable life after a slight decrease in the first several cycles. The reason for the selfenhancement could be ascribed to the strong interphase interaction between MnO and graphene flakes. Our work provides a new understanding and insight for the electrochemical behavior of composite electrodes in LIBs and is helpful for the fabrication of high-performance anode materials. & 2014 Elsevier Ltd. All rights reserved.
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Corresponding author. Tel./fax: +86 10 6443 4916. E-mail address: [email protected] (H. Song). 1 These authors contributed equally to this work. http://dx.doi.org/10.1016/j.nanoen.2014.09.012 2211-2855/& 2014 Elsevier Ltd. All rights reserved.
Enhanced electrochemical performance
Introduction In common sense, long-term cycling of the lithium ion batteries (LIBs) especially under large current densities causes the capacity fading because the structure of electrode materials will collapse gradually by severe volume effect during lithiation/delithiation [1–7]. This volume effect are even more remarkably in large capacitive active components such as Si, Sn, and some transition metal oxides in which rapid capacity loss in few cycles has been the great obstacle for their practical application, especially for high power supplies. To overcome this great problem, kinds of attempts have been taken principally to buffer the volume changes thus keep the structural integrity of the electrode materials. For example, nanosized hollow [8,9] or wire-like structures [10–12] were reported to have effectively buffering effects as well as shortening the path of Li + diffusion, which could improve the cycle life and rate performance of the electrode materials. Regarding better volume effect inhibition and faster charge transfer, various delicate structures of metal oxides/carbon composites were elaborately fabricated such as core–shell [13–16], encapsulation [17,18], and coaxial nanocable structures [19,20]. In recent years, masses of graphene/metal oxide composites were also developed along with the rising of graphene [21–26]. Although the cyclic capability was improved to some extent for most of the obtained electrodes, their capacities still decreased under the increased cycle times. In a few of the previous reports, though favorable capacity enhancement found in some electrode materials and several possible mechanisms were proposed, unfortunately, the intrinsic mechanism is still confused so far. Sun et al. [26] and Li et al. [27] owed the improvement to the formation of ultra-fine metal oxide particles from metal oxide on graphene sheets. In another work, Li and coworkers attributed the capacity enhancement of Fe2O3 nano-network to the formation of additional reversible polymer/gel films accompanied with the increased surface area [28]. Cui et al. proposed the activation mechanism by nitride [29]. Herein, we report the remarkable in-situ capacity enhancement during cycling in a novel MnO nanowire/ graphene nanosheet (MnO/GNS) composite anode in LIBs. Moreover, the morphologies and structural changes of MnO/ GNS after various charge/discharge cycles were investigated in detail by X-ray diffraction (XRD), high resolution transmission electron microscopy (HRTEM), and Fourier transfer infrared spectroscopy (FT-IR). The pristine nanowire structure of MnO was gradually pulverized with the simultaneous formation of smaller MnO nano-spindles and further nanoparticles, but these newly formed nanoparticles still attached on graphene sheets noticeably without the separation. As a consequence, it was put forward strong
173 interphase interaction between MnO and graphene can efficiently trap the newly formed nanoparticles and smaller nanoparticles promoted faster Li + exchange as well as the charge transfer between graphene sheets, thus improved the electrochemical performances. Our work provides a new horizon and deep understanding for advancing the design of oxide based composite materials for LIB electrodes.
Experimental section Sample preparation MnO/GNS composite was prepared via a two step method (Figure 1): preparation of MnO2/graphene oxide (GO) based on a modified Hummers' method [30] and a following thermal treatment in N2 atmosphere. Detailedly, 2.5 g of natural graphite powder (ca. 150 μm), 1.25 g of NaNO3 and 120 mL of concentrated H2SO4 (98 wt%) were firstly mixed in a 250 mL round-bottom flask with stirring in ice-water bath, and then 10 g of KMnO4 was gradually added in 5 min. The mixture was stirred in the ice-water bath for 30 min and then kept in 35 1C water bath for 2 h. After that, the mixture was diluted by 500 mL de-ionized water (DI-water, 18.2 MΩ) and then dialyzed in DI-water for 3 days until the pH reached to 7. The product was dried at 80 1C where the MnO2/GO powder was achieved. MnO/GNS composite was fabricated by annealing the obtained MnO2/GO at 400 1C under the protection of N2 for 2 h. For comparison, GNSs were also prepared. Briefly, before the step of dialysis during the preparation of MnO2/GO composite, excess hydrogen peroxide (30 wt%) was added to totally remove MnO2 and pure GO was thus attained. Then, GNSs were obtained by annealing GO also at 400 1C for 2 h in N2 atmosphere. Another comparative sample of free MnO nanoparticles were prepared by reducing previously reported α-MnO2 in H2 flow at 400 1C for 2 h [31].
Characterization and electrochemical measurements The obtained samples were characterized by X-ray diffraction (XRD, Rigaku D/max-2500B2 + /PCX, Cu Kα, λ= 1.54056 Å), transmission electron microscopy (TEM, HT800, Hitachi), high resolution transmission electron microscopy (HRTEM, JEOL 3010, JEOL) and thermogravimetric analysis (TGA, STA449C Jupiter, NETZSCH). The electrochemical measurements were tested in CR2032 (3 V) coin-type cell as described in previous work of our group [32]. For preparing working electrodes, a mixture of active sample (MnO/GNS, GNS or pure MnO, 80 wt%), PolyVinylidene DiFluoride (binder, 10 wt%) and acetylene black (10 wt%)
Figure 1 Schematic description for the formation of MnO/GNS composite.
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were coated on the circular foam nickel plates (ca. 1.5 cm2). The mass of the active component for each plate is about 5 mg (ca. 3–4 mg cm 2). 1 M LiPF6 solution in the mixture of ethylene carbonate and dimethyl carbonate (v/v, 1:1) was employed as electrolyte. After assembling, the cells were charged and discharged in Land CT2001A system at various current densities in the voltage ranging from 0.01 to 3.0 V. Cyclic voltammograms (CV) were carried out on a CHI660B workstation with the testing voltage between 3.0 and 0.01 V at a scan rate of 0.1 mV s 1. The electrochemical impedance spectroscopy (EIS) of MnO/GNS at different cycles was taken on an electrochemical workstation (CHI660B) using the frequency response analysis. The impedance spectra were obtained by applying a sine wave with amplitude of 5.0 mV over the frequency range
Figure 2 XRD patterns of as-prepared GO, MnO2/GO and MnO/ GNS composite, the TG curve of MnO/GNS is inserted at upper left.
from 100 kHz to 0.01 Hz. Fitting of impedance spectra to the proposed equivalent circuit was performed by the code Zview.
Results and discussion Two main MnO2 crystal structures, α-MnO2 (JCPDS no. 440141) and γ-MnO2 (JCPDS no. 44-0142), can be easily distinguished from the broad and dispersive peaks in the red curve of MnO2/GO composite (Figure 2). While after annealing, the (001) peak of GO [33] vanishes with the appearance of a weak and dispersive peak around 261 in the black line of MnO/GNS, which is in accordance with the (002) peak of carbon. Besides, the residual three sharp diffraction peaks in MnO/GNS are well indexed to cubic MnO (JCPDS no. 07-0230), which means that two different MnO2 nanowires are totally transferred to MnO with an enhanced crystalline degree after annealing. The mass content of MnO in MnO/GNS composite is 64.4% measured by TG analysis at air atmosphere on the basis of the completed burning up of GNSs and MnOx in the form of Mn2O3 at 900 1C [34,35]. Figure 3 shows the micro-morphologies of the resulted composites. Nano-sized MnO2 “particles” are uniformly anchored on the thin-layered GO sheets (Figure 3a). From the magnified images (Figure 3b and c), it can be found that those “particles” are actually built up by aloe-like MnO2 nanowire clusters with the diameter of ca. 10 nm. After heating treatment, MnO2/GO is transferred to MnO/GNS composite with the well preserved morphology. The well crystalline MnO nanowires exhibit the lengths of 50–400 nm and the diameters of 5–15 nm (Figure 3e and f). The electrochemical performances of the MnO/GNS composite as anode for LIBs were carried out and the results are
Figure 3 TEM images of MnO2/GO (a–c) and MnO/GNS composite (d, e), and HRTEM images of MnO/GNS (f).
Enhanced electrochemical performance
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Figure 4 (a) CV curves of the MnO/GNS composite as the anode material for LIBs; (b) charge–discharge processes of MnO/GNS; (c) cyclic performances of MnO/GNS, GNS, and pure MnO under different current densities; (d) cyclic performance under the current of 500 mA g 1; (e) differential capacity vs. cell voltage plots and (f) EIS spectra of MnO/GNS for different cycles under the current density of 500 mA g 1, the inserted figure in (f) is the zoomed area marked by yellow box; (g) Randles equivalent circuit model of the LIB.
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Table 1 Simulation results of the kinetic parameters of MnO/GNS.
1st 30th 60th 135th
Re (Ω)
Rf (Ω)
Rct (Ω)
5.108 5.173 5.306 6.561
4.265 3.965 4.345 4.463
9.118 8.591 9.306 6.241
given in Figure 4. The first three cycles of CV are shown in Figure 4a. The large reduction peak at about 0.12 V in the first cycle can be divided into two parts: the irreversible part which Li + is consumed by reaction with carbon and the reversible part which Mn2 + is reduced to Mn0. The reduction peak of Mn2 + to Mn0 is shifted to 0.30 V in the subsequent cycles due to the improved kinetics of the MnO/GNS electrode during first lithiation [26,36]. The wave concern at around 0.7 V is due to the reduction of high oxidation state Mn (small amount of impurity) by lithium and the
Figure 5 HRTEM images of MnO/GNS (a) original, (b) cycling for 1 time, (c and d) 30 times and (e and f) 100 times at 500 mA g
1
.
Enhanced electrochemical performance formation of solid electrolyte interphase (SEI) films, the peak located at ca. 1.37 V can be attributed to the reduction of the electrolyte [37–39]. The main anodic peak is located at ca. 1.30 V, corresponding to the oxidation of Mn0 to Mn2 + , and the weak and broad peak at around 2.10 V is associated with the oxidation of Mn2 + to a higher oxidation state [14,26]. The detailed charge–discharge process of the MnO/GNS is given in Figure 4b. It shows good accordance with the results of CV tests. The rate capabilities of the obtained materials were measured by galvanostatic charge–discharge measurements at different current densities. The reversible capacities of the MnO/GNS composite are observed to be the highest compared with other obtained materials even the current density is successively increased to 0.1, 0.2, 0.5, and 1.0 A g 1. Noteworthily, the specific capacity of the MnO/GNS reaches to a higher value of 860 mAh g 1 than that of in the first several cycles when the current density decreases back to 50 mA g 1 after the 50th cycle. For the clear observation of the capacity enhancement phenomenon, the MnO/GNS were cycled at the large current density of 500 mA g 1 for adequate cycles. The result given in Figure 4d shows that the reversible capacity firstly fades from 630 mAh g 1 to ca. 510 mAh g 1 in the initial 30 cycles, then gradually increases to ca. 930 mAh g 1 after the subsequent 470 cycles. The differential plots of capacity vs. cell voltage presents in Figure 4e indicates the Li storage capability of MnO/GNS is mainly at the voltage of ca. 0.4 V. The EIS spectra of MnO/GNS at different cycles under the current density of 500 mA g 1 are given in Figure 4f. According to the equivalent circuit model shown in Figure 4g [40,41], the simulation results of the kinetic parameters are presented in Table 1. Re is the electrolyte resistance, and Rf and Cf are the resistance and capacitance of the solid-state interface layer formed on the surface of the electrodes, respectively. Cdl and Rct are the double-layer capacitance and chargetransfer resistance, respectively. Zw is the Warburg impedance related to the diffusion of lithium ions into the bulk of the electrode. The Rct is obviously decreased after 135 cycles, indicating the dramatic enhanced electrochemical activity of MnO/GNS along with cycling. This special and favorable phenomenon has also been reported in several graphene/metal oxides composites (e.g., Fe3O4 [42], Co3O4 [43], MnO [26,44] and CuO [45]). Though some explanations such as formation of the ultrafine nanoparticles or reversible polymer/gel films were proposed previously as we mentioned in the introduction part, the detailed and certificated clarification is still urgently required. In this case, the micro-morphologies of the MnO/GNS with different cycles were investigated by HRTEM and the results are presented in Figure 5. The MnO began to collapse in the first cycle (Figure 5b). After cycling for 30 times, the nanowires are transformed into smaller nano-spindles with the diameters of ca. 5–10 nm (Figure 5c and d). At this time, the MnO/GNS composite performed the lowest specific capacity of 510 mAh g 1. With the continuous cycling, these nano-spindles further collapse to nanoparticles (Figure 5e and f), but the graphene-supported structure still remains unchanged. On the whole, though MnO nanowires were breakdown to smaller MnO nanoparticles during cycling, the ultrafine MnO still homogeneously and tightly anchored on the graphene platelets without detachment. And the smaller
177 MnO nanoparticles are deduced to endow better electrical contact with graphene and superior cycle stability [26]. The structural variation and integrity of the electrodes after different cycles were measured by XRD and the patterns given in Figure 6 can also express the hold of MnO active part in MnO/GNS during cycling. The diffraction peaks of MnO become weak and dispersive in the XRD patterns after the first charge–discharge process, which supplied more evidences that well crystalline nanowires pulverized to smaller nanostructures. The smaller MnO nanoparticles exhibits a much more stable structure because other XRD curves from different cycles are the same as that of the first cycle, and those MnO peaks can still be indexed to the original cubic MnO (JCPDS no. 07-0230). What's more, the similar XRD patterns obtained from 1st to 500th cycles of the electrode indicates that, although MnO nanowires are collapsed in the beginning, the active part does not drop out of the current collector. From the mentioned above, the enhanced electrochemical performance of the obtained MnO/GNS is much dependent on its temporal structure. It cannot be ascribed to the initially delicate morphology for the reason that aloe-like MnO nanowires are collapsed in the first several cycles. According to the previous reports [1–7], pulverization of metal oxides leads to the separation of active components from the electrodes. Therefore, a capacity fading curve was put forward as the short-dash line (1) shown in Figure 4d. On the other hand, metal oxides with ultrafine particle sizes can promote Li + migration during lithiation/delithiation. When the smaller nanoparticles are anchored on graphene sheets, much closer connection can also provide better electronic conductivity. These improved electrochemical connection and electrolyte access produce enhanced rate capability of the composite material which is seen in the curve (2) of Figure 4d. The morphology change of the MnO/ GNS is much consistent with its electrochemical property, which allows us to deduce that the electrochemical property enhancement of the obtained MnO/GNS is superimposed of these two effects. Gradual pulverization occurs during cycling and small amount of the MnO is separated from the current collector inevitably, which causes the capacity fading of the MnO/GNS composite in the first several cycles. However, tight anchoring of the subsequently formed smaller MnO component from the further collapse of MnO nanowires can contribute to improve the
Figure 6 XRD patterns of MnO/GNS electrodes after cycling for various cycles (1–500 times).
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Figure 7
FT-IR patterns of GO, MnO2/GO, GNS, and MnO/GNS.
kinetic process of the electrode thus gain the capacity selfenhancement. Then, how can this complex trapping procedure be achieved in material designing? From the FT-IR measurements (Figure 7), we observed a strong interaction between MnO and graphene nanosheets in the pristine MnO/GNS composite. Comparing the curves of GO and GNS (obtained by totally removing MnO from MnO/GNS), the peaks at ca. 1050, 1220, 1400 cm 1 and the broad peak ranging from 3700 cm 1 to 2000 cm 1 in GO disappear after annealing, which indicates the decomposition of most alkoxy, hydroxyl, epoxy, and carboxyl [46,47]. The remained sharp peak at 3450 cm 1 in both GNS and MnO/GNS is attributed to the inevitable absorbed water. Furthermore, it should be noticed that: (1) the C = O peak at 1730 cm 1 cannot be observed in both (patterns of) MnO2/ GO and MnO/GNS, while it appears after the removal of MnOx; (2) the C–O–C peak (ca. 1220 cm 1) is vanished with the anchored of MnO2 in MnO2/GO, while, it exhibits in both GNS and MnO/GNS; (3) the alkoxy C–O peak in GO (ca. 1049 cm 1) is broadened at ca. 1088 cm 1 in MnO2/GO, while after annealing, this peak in MnO/GNS becomes sharper and shifted to higher wave number of ca. 1113 cm 1. As a result, strong physicochemical interaction between MnO and GNS is proposed due to the formation of Mn–O–C bonds after annealing [42,48]. Though the main reasons for the capacity enhancement were still not found due to the component complexity of the electrode materials after charge–discharge process, summarizing various typical reports, except the influence of novel nanostructures (e.g., yolk shell structure [49]), we speculate that achieving strong interphase interactions would also be responsible for the capacity enhancement. In graphene composite materials, the capacity enhancement is mainly found when the materials are suffered a thermal treatment at a relative high temperature, where intensive physicochemical interactions are built up in this case [42–45,50,51]. In some of the graphene encapsulated structures, strong electrostatic force between graphene and metal oxides is always required as the induction prerequisite in the fabrication procedure [25,52]. On the other
hand, the capacity enhancement can be barely found in the electrode materials prepared from soft methods such as chemical deposition [53–55] and solvothermal method [56,57] which the strong interphase interactions hardly achieve in these approaches. With this strong interaction, the shedding of pulverized nanoparticles from GNS plane and even electronic collector is largely suppressed, and the electrical conductivity can be well enhanced, which are obviously beneficial for the improvement of electrochemical properties.
Conclusion Special in-situ Li + storage capacity enhancement during cycling was observed in a well designed MnO nanowire/GNS composite. During cycling, the capacity was dropped in the first several cycles and then increased significantly, while the MnO nanowires were gradually collapsed to smaller nano-spindles and further ultrafine nanoparticles. But these newly formed nanoparticles were still tightly captured by graphene sheets without separation. The self-enhancement in this case was attributed to the improved Li + accessibility and electronic conductivity by the trapped smaller nanoparticles.
Acknowledgments: This work was supported by the National Natural Science Foundation of China (51202009 and 51272016), and Foundation of Excellent Doctoral Dissertation of Beijing City (YB20121001001).
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Su Zhang received his bachelor degree in Materials Science and Engineering from Beijing University of Chemical Technology in 2011. Now Su Zhang is pursuing his Ph.D. degree in Chemical Engineering and Technology in Beijing University of Chemical Technology under the guidance of Prof. Huaihe Song. His current research interest is on low dimensional carbon materials and energy storage devices.
Lingxiang Zhu received his bachelor degree in Materials Science and Engineering from Beijing University of Chemical Technology in 2012, and then worked as Research Assistant in Dr. Huaihe Song's group until 2013. Now Lingxiang is pursuing his Ph.D. degree in Chemical Engineering in University at Buffalo, The State University of New York. His current research interest is on graphene based materials as well as membranes for CO2 capture.
180 Huaihe Song obtained his Ph.D. degree in Chemical Processing from the Institute of Coal Chemistry, Chinese Academy of Sciences, China, in 1997. After two years of Postdoc Research in Beijing University of Chemical Technology (BUCT), he then became a Professor in College of Materials Science and Engineering, BUCT, from 2001. His main research interests are in the fields of carbon and carbon-based composite materials, including pitch-based carbon materials, carbon nanomaterials, ordered mesoporous carbons and carbon materials for energy storage, such as in lithium-ion batteries and supercapacitors. He has published more than 220 peer-reviewed papers up to now. Xiaohong Chen received her Bachelor degree in Taiyuan University of Technology in 1992 and Ph.D. degree from Beijing University of Chemical Technology in 1998. After two years of Postdoc Research in Max Planck Institute for Solid State Research, she joined Beijing University of Chemical Technology as a Professor in 2005. Her current research interests are mainly focusing on carbon nanomaterials, aerogel and bamboo charcoal. She has published more than 50 articles up to now.
S. Zhang et al. Jisheng Zhou is currently a lecturer in the Materials Science and Engineering College, Beijing University of Chemical Technology, China. He received his B.E. in Chemical Engineering from China University of Petroleum in 2005, and Ph.D. in Materials Science and Engineering from Beijing University of Chemical Technology in 2011. His research interests focuses on synthesis of nanocarbons and nanocarbon-based composites, and their applications in energy storage.
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journal homepage: www.elsevier.com/locate/nanoenergy
RAPID COMMUNICATION
Enhanced electrochemical performance of MnO nanowire/graphene composite during cycling as the anode material for lithium-ion batteries Su Zhang1, Lingxiang Zhu1, Huaihe Songn, Xiaohong Chen, Jisheng Zhou State Key Laboratory of Chemical Resource Engineering, Beijing Key Laboratory of Electrochemical Process and Technology for Materials, Beijing University of Chemical Technology, Beijing 100029, PR China Received 27 June 2014; received in revised form 7 September 2014; accepted 11 September 2014 Available online 22 September 2014
KEYWORDS
Abstract
Graphene; MnO nanowire; Lithium ion batteries; Capacity enhancement; Interphase interaction
Generally, the capacity of lithium-ion batteries (LIBs) will fade gradually during cycling especially under large current densities because of the structural collapse of electrode materials. Herein, dramatic and favorable electrochemical performance enhancement was observed in a simply achieved MnO nanowire/graphene composite during long-term cycling when utilized as the anode material for LIBs. Characterized by high-resolution transmission electron microscopy, X-ray diffraction and Fourier-transfer infrared spectroscopy, the MnO nanowires were gradually collapsed to nano-spindles and further nanoparticles but these newly formed nanoparticles are still stably anchored on the graphene lamellas. Correspondingly, the specific capacity of the MnO nanowire/graphene electrode exhibited a significant enhancement with a durable life after a slight decrease in the first several cycles. The reason for the selfenhancement could be ascribed to the strong interphase interaction between MnO and graphene flakes. Our work provides a new understanding and insight for the electrochemical behavior of composite electrodes in LIBs and is helpful for the fabrication of high-performance anode materials. & 2014 Elsevier Ltd. All rights reserved.
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Corresponding author. Tel./fax: +86 10 6443 4916. E-mail address: [email protected] (H. Song). 1 These authors contributed equally to this work. http://dx.doi.org/10.1016/j.nanoen.2014.09.012 2211-2855/& 2014 Elsevier Ltd. All rights reserved.
Enhanced electrochemical performance
Introduction In common sense, long-term cycling of the lithium ion batteries (LIBs) especially under large current densities causes the capacity fading because the structure of electrode materials will collapse gradually by severe volume effect during lithiation/delithiation [1–7]. This volume effect are even more remarkably in large capacitive active components such as Si, Sn, and some transition metal oxides in which rapid capacity loss in few cycles has been the great obstacle for their practical application, especially for high power supplies. To overcome this great problem, kinds of attempts have been taken principally to buffer the volume changes thus keep the structural integrity of the electrode materials. For example, nanosized hollow [8,9] or wire-like structures [10–12] were reported to have effectively buffering effects as well as shortening the path of Li + diffusion, which could improve the cycle life and rate performance of the electrode materials. Regarding better volume effect inhibition and faster charge transfer, various delicate structures of metal oxides/carbon composites were elaborately fabricated such as core–shell [13–16], encapsulation [17,18], and coaxial nanocable structures [19,20]. In recent years, masses of graphene/metal oxide composites were also developed along with the rising of graphene [21–26]. Although the cyclic capability was improved to some extent for most of the obtained electrodes, their capacities still decreased under the increased cycle times. In a few of the previous reports, though favorable capacity enhancement found in some electrode materials and several possible mechanisms were proposed, unfortunately, the intrinsic mechanism is still confused so far. Sun et al. [26] and Li et al. [27] owed the improvement to the formation of ultra-fine metal oxide particles from metal oxide on graphene sheets. In another work, Li and coworkers attributed the capacity enhancement of Fe2O3 nano-network to the formation of additional reversible polymer/gel films accompanied with the increased surface area [28]. Cui et al. proposed the activation mechanism by nitride [29]. Herein, we report the remarkable in-situ capacity enhancement during cycling in a novel MnO nanowire/ graphene nanosheet (MnO/GNS) composite anode in LIBs. Moreover, the morphologies and structural changes of MnO/ GNS after various charge/discharge cycles were investigated in detail by X-ray diffraction (XRD), high resolution transmission electron microscopy (HRTEM), and Fourier transfer infrared spectroscopy (FT-IR). The pristine nanowire structure of MnO was gradually pulverized with the simultaneous formation of smaller MnO nano-spindles and further nanoparticles, but these newly formed nanoparticles still attached on graphene sheets noticeably without the separation. As a consequence, it was put forward strong
173 interphase interaction between MnO and graphene can efficiently trap the newly formed nanoparticles and smaller nanoparticles promoted faster Li + exchange as well as the charge transfer between graphene sheets, thus improved the electrochemical performances. Our work provides a new horizon and deep understanding for advancing the design of oxide based composite materials for LIB electrodes.
Experimental section Sample preparation MnO/GNS composite was prepared via a two step method (Figure 1): preparation of MnO2/graphene oxide (GO) based on a modified Hummers' method [30] and a following thermal treatment in N2 atmosphere. Detailedly, 2.5 g of natural graphite powder (ca. 150 μm), 1.25 g of NaNO3 and 120 mL of concentrated H2SO4 (98 wt%) were firstly mixed in a 250 mL round-bottom flask with stirring in ice-water bath, and then 10 g of KMnO4 was gradually added in 5 min. The mixture was stirred in the ice-water bath for 30 min and then kept in 35 1C water bath for 2 h. After that, the mixture was diluted by 500 mL de-ionized water (DI-water, 18.2 MΩ) and then dialyzed in DI-water for 3 days until the pH reached to 7. The product was dried at 80 1C where the MnO2/GO powder was achieved. MnO/GNS composite was fabricated by annealing the obtained MnO2/GO at 400 1C under the protection of N2 for 2 h. For comparison, GNSs were also prepared. Briefly, before the step of dialysis during the preparation of MnO2/GO composite, excess hydrogen peroxide (30 wt%) was added to totally remove MnO2 and pure GO was thus attained. Then, GNSs were obtained by annealing GO also at 400 1C for 2 h in N2 atmosphere. Another comparative sample of free MnO nanoparticles were prepared by reducing previously reported α-MnO2 in H2 flow at 400 1C for 2 h [31].
Characterization and electrochemical measurements The obtained samples were characterized by X-ray diffraction (XRD, Rigaku D/max-2500B2 + /PCX, Cu Kα, λ= 1.54056 Å), transmission electron microscopy (TEM, HT800, Hitachi), high resolution transmission electron microscopy (HRTEM, JEOL 3010, JEOL) and thermogravimetric analysis (TGA, STA449C Jupiter, NETZSCH). The electrochemical measurements were tested in CR2032 (3 V) coin-type cell as described in previous work of our group [32]. For preparing working electrodes, a mixture of active sample (MnO/GNS, GNS or pure MnO, 80 wt%), PolyVinylidene DiFluoride (binder, 10 wt%) and acetylene black (10 wt%)
Figure 1 Schematic description for the formation of MnO/GNS composite.
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were coated on the circular foam nickel plates (ca. 1.5 cm2). The mass of the active component for each plate is about 5 mg (ca. 3–4 mg cm 2). 1 M LiPF6 solution in the mixture of ethylene carbonate and dimethyl carbonate (v/v, 1:1) was employed as electrolyte. After assembling, the cells were charged and discharged in Land CT2001A system at various current densities in the voltage ranging from 0.01 to 3.0 V. Cyclic voltammograms (CV) were carried out on a CHI660B workstation with the testing voltage between 3.0 and 0.01 V at a scan rate of 0.1 mV s 1. The electrochemical impedance spectroscopy (EIS) of MnO/GNS at different cycles was taken on an electrochemical workstation (CHI660B) using the frequency response analysis. The impedance spectra were obtained by applying a sine wave with amplitude of 5.0 mV over the frequency range
Figure 2 XRD patterns of as-prepared GO, MnO2/GO and MnO/ GNS composite, the TG curve of MnO/GNS is inserted at upper left.
from 100 kHz to 0.01 Hz. Fitting of impedance spectra to the proposed equivalent circuit was performed by the code Zview.
Results and discussion Two main MnO2 crystal structures, α-MnO2 (JCPDS no. 440141) and γ-MnO2 (JCPDS no. 44-0142), can be easily distinguished from the broad and dispersive peaks in the red curve of MnO2/GO composite (Figure 2). While after annealing, the (001) peak of GO [33] vanishes with the appearance of a weak and dispersive peak around 261 in the black line of MnO/GNS, which is in accordance with the (002) peak of carbon. Besides, the residual three sharp diffraction peaks in MnO/GNS are well indexed to cubic MnO (JCPDS no. 07-0230), which means that two different MnO2 nanowires are totally transferred to MnO with an enhanced crystalline degree after annealing. The mass content of MnO in MnO/GNS composite is 64.4% measured by TG analysis at air atmosphere on the basis of the completed burning up of GNSs and MnOx in the form of Mn2O3 at 900 1C [34,35]. Figure 3 shows the micro-morphologies of the resulted composites. Nano-sized MnO2 “particles” are uniformly anchored on the thin-layered GO sheets (Figure 3a). From the magnified images (Figure 3b and c), it can be found that those “particles” are actually built up by aloe-like MnO2 nanowire clusters with the diameter of ca. 10 nm. After heating treatment, MnO2/GO is transferred to MnO/GNS composite with the well preserved morphology. The well crystalline MnO nanowires exhibit the lengths of 50–400 nm and the diameters of 5–15 nm (Figure 3e and f). The electrochemical performances of the MnO/GNS composite as anode for LIBs were carried out and the results are
Figure 3 TEM images of MnO2/GO (a–c) and MnO/GNS composite (d, e), and HRTEM images of MnO/GNS (f).
Enhanced electrochemical performance
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Figure 4 (a) CV curves of the MnO/GNS composite as the anode material for LIBs; (b) charge–discharge processes of MnO/GNS; (c) cyclic performances of MnO/GNS, GNS, and pure MnO under different current densities; (d) cyclic performance under the current of 500 mA g 1; (e) differential capacity vs. cell voltage plots and (f) EIS spectra of MnO/GNS for different cycles under the current density of 500 mA g 1, the inserted figure in (f) is the zoomed area marked by yellow box; (g) Randles equivalent circuit model of the LIB.
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Table 1 Simulation results of the kinetic parameters of MnO/GNS.
1st 30th 60th 135th
Re (Ω)
Rf (Ω)
Rct (Ω)
5.108 5.173 5.306 6.561
4.265 3.965 4.345 4.463
9.118 8.591 9.306 6.241
given in Figure 4. The first three cycles of CV are shown in Figure 4a. The large reduction peak at about 0.12 V in the first cycle can be divided into two parts: the irreversible part which Li + is consumed by reaction with carbon and the reversible part which Mn2 + is reduced to Mn0. The reduction peak of Mn2 + to Mn0 is shifted to 0.30 V in the subsequent cycles due to the improved kinetics of the MnO/GNS electrode during first lithiation [26,36]. The wave concern at around 0.7 V is due to the reduction of high oxidation state Mn (small amount of impurity) by lithium and the
Figure 5 HRTEM images of MnO/GNS (a) original, (b) cycling for 1 time, (c and d) 30 times and (e and f) 100 times at 500 mA g
1
.
Enhanced electrochemical performance formation of solid electrolyte interphase (SEI) films, the peak located at ca. 1.37 V can be attributed to the reduction of the electrolyte [37–39]. The main anodic peak is located at ca. 1.30 V, corresponding to the oxidation of Mn0 to Mn2 + , and the weak and broad peak at around 2.10 V is associated with the oxidation of Mn2 + to a higher oxidation state [14,26]. The detailed charge–discharge process of the MnO/GNS is given in Figure 4b. It shows good accordance with the results of CV tests. The rate capabilities of the obtained materials were measured by galvanostatic charge–discharge measurements at different current densities. The reversible capacities of the MnO/GNS composite are observed to be the highest compared with other obtained materials even the current density is successively increased to 0.1, 0.2, 0.5, and 1.0 A g 1. Noteworthily, the specific capacity of the MnO/GNS reaches to a higher value of 860 mAh g 1 than that of in the first several cycles when the current density decreases back to 50 mA g 1 after the 50th cycle. For the clear observation of the capacity enhancement phenomenon, the MnO/GNS were cycled at the large current density of 500 mA g 1 for adequate cycles. The result given in Figure 4d shows that the reversible capacity firstly fades from 630 mAh g 1 to ca. 510 mAh g 1 in the initial 30 cycles, then gradually increases to ca. 930 mAh g 1 after the subsequent 470 cycles. The differential plots of capacity vs. cell voltage presents in Figure 4e indicates the Li storage capability of MnO/GNS is mainly at the voltage of ca. 0.4 V. The EIS spectra of MnO/GNS at different cycles under the current density of 500 mA g 1 are given in Figure 4f. According to the equivalent circuit model shown in Figure 4g [40,41], the simulation results of the kinetic parameters are presented in Table 1. Re is the electrolyte resistance, and Rf and Cf are the resistance and capacitance of the solid-state interface layer formed on the surface of the electrodes, respectively. Cdl and Rct are the double-layer capacitance and chargetransfer resistance, respectively. Zw is the Warburg impedance related to the diffusion of lithium ions into the bulk of the electrode. The Rct is obviously decreased after 135 cycles, indicating the dramatic enhanced electrochemical activity of MnO/GNS along with cycling. This special and favorable phenomenon has also been reported in several graphene/metal oxides composites (e.g., Fe3O4 [42], Co3O4 [43], MnO [26,44] and CuO [45]). Though some explanations such as formation of the ultrafine nanoparticles or reversible polymer/gel films were proposed previously as we mentioned in the introduction part, the detailed and certificated clarification is still urgently required. In this case, the micro-morphologies of the MnO/GNS with different cycles were investigated by HRTEM and the results are presented in Figure 5. The MnO began to collapse in the first cycle (Figure 5b). After cycling for 30 times, the nanowires are transformed into smaller nano-spindles with the diameters of ca. 5–10 nm (Figure 5c and d). At this time, the MnO/GNS composite performed the lowest specific capacity of 510 mAh g 1. With the continuous cycling, these nano-spindles further collapse to nanoparticles (Figure 5e and f), but the graphene-supported structure still remains unchanged. On the whole, though MnO nanowires were breakdown to smaller MnO nanoparticles during cycling, the ultrafine MnO still homogeneously and tightly anchored on the graphene platelets without detachment. And the smaller
177 MnO nanoparticles are deduced to endow better electrical contact with graphene and superior cycle stability [26]. The structural variation and integrity of the electrodes after different cycles were measured by XRD and the patterns given in Figure 6 can also express the hold of MnO active part in MnO/GNS during cycling. The diffraction peaks of MnO become weak and dispersive in the XRD patterns after the first charge–discharge process, which supplied more evidences that well crystalline nanowires pulverized to smaller nanostructures. The smaller MnO nanoparticles exhibits a much more stable structure because other XRD curves from different cycles are the same as that of the first cycle, and those MnO peaks can still be indexed to the original cubic MnO (JCPDS no. 07-0230). What's more, the similar XRD patterns obtained from 1st to 500th cycles of the electrode indicates that, although MnO nanowires are collapsed in the beginning, the active part does not drop out of the current collector. From the mentioned above, the enhanced electrochemical performance of the obtained MnO/GNS is much dependent on its temporal structure. It cannot be ascribed to the initially delicate morphology for the reason that aloe-like MnO nanowires are collapsed in the first several cycles. According to the previous reports [1–7], pulverization of metal oxides leads to the separation of active components from the electrodes. Therefore, a capacity fading curve was put forward as the short-dash line (1) shown in Figure 4d. On the other hand, metal oxides with ultrafine particle sizes can promote Li + migration during lithiation/delithiation. When the smaller nanoparticles are anchored on graphene sheets, much closer connection can also provide better electronic conductivity. These improved electrochemical connection and electrolyte access produce enhanced rate capability of the composite material which is seen in the curve (2) of Figure 4d. The morphology change of the MnO/ GNS is much consistent with its electrochemical property, which allows us to deduce that the electrochemical property enhancement of the obtained MnO/GNS is superimposed of these two effects. Gradual pulverization occurs during cycling and small amount of the MnO is separated from the current collector inevitably, which causes the capacity fading of the MnO/GNS composite in the first several cycles. However, tight anchoring of the subsequently formed smaller MnO component from the further collapse of MnO nanowires can contribute to improve the
Figure 6 XRD patterns of MnO/GNS electrodes after cycling for various cycles (1–500 times).
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Figure 7
FT-IR patterns of GO, MnO2/GO, GNS, and MnO/GNS.
kinetic process of the electrode thus gain the capacity selfenhancement. Then, how can this complex trapping procedure be achieved in material designing? From the FT-IR measurements (Figure 7), we observed a strong interaction between MnO and graphene nanosheets in the pristine MnO/GNS composite. Comparing the curves of GO and GNS (obtained by totally removing MnO from MnO/GNS), the peaks at ca. 1050, 1220, 1400 cm 1 and the broad peak ranging from 3700 cm 1 to 2000 cm 1 in GO disappear after annealing, which indicates the decomposition of most alkoxy, hydroxyl, epoxy, and carboxyl [46,47]. The remained sharp peak at 3450 cm 1 in both GNS and MnO/GNS is attributed to the inevitable absorbed water. Furthermore, it should be noticed that: (1) the C = O peak at 1730 cm 1 cannot be observed in both (patterns of) MnO2/ GO and MnO/GNS, while it appears after the removal of MnOx; (2) the C–O–C peak (ca. 1220 cm 1) is vanished with the anchored of MnO2 in MnO2/GO, while, it exhibits in both GNS and MnO/GNS; (3) the alkoxy C–O peak in GO (ca. 1049 cm 1) is broadened at ca. 1088 cm 1 in MnO2/GO, while after annealing, this peak in MnO/GNS becomes sharper and shifted to higher wave number of ca. 1113 cm 1. As a result, strong physicochemical interaction between MnO and GNS is proposed due to the formation of Mn–O–C bonds after annealing [42,48]. Though the main reasons for the capacity enhancement were still not found due to the component complexity of the electrode materials after charge–discharge process, summarizing various typical reports, except the influence of novel nanostructures (e.g., yolk shell structure [49]), we speculate that achieving strong interphase interactions would also be responsible for the capacity enhancement. In graphene composite materials, the capacity enhancement is mainly found when the materials are suffered a thermal treatment at a relative high temperature, where intensive physicochemical interactions are built up in this case [42–45,50,51]. In some of the graphene encapsulated structures, strong electrostatic force between graphene and metal oxides is always required as the induction prerequisite in the fabrication procedure [25,52]. On the other
hand, the capacity enhancement can be barely found in the electrode materials prepared from soft methods such as chemical deposition [53–55] and solvothermal method [56,57] which the strong interphase interactions hardly achieve in these approaches. With this strong interaction, the shedding of pulverized nanoparticles from GNS plane and even electronic collector is largely suppressed, and the electrical conductivity can be well enhanced, which are obviously beneficial for the improvement of electrochemical properties.
Conclusion Special in-situ Li + storage capacity enhancement during cycling was observed in a well designed MnO nanowire/GNS composite. During cycling, the capacity was dropped in the first several cycles and then increased significantly, while the MnO nanowires were gradually collapsed to smaller nano-spindles and further ultrafine nanoparticles. But these newly formed nanoparticles were still tightly captured by graphene sheets without separation. The self-enhancement in this case was attributed to the improved Li + accessibility and electronic conductivity by the trapped smaller nanoparticles.
Acknowledgments: This work was supported by the National Natural Science Foundation of China (51202009 and 51272016), and Foundation of Excellent Doctoral Dissertation of Beijing City (YB20121001001).
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Su Zhang received his bachelor degree in Materials Science and Engineering from Beijing University of Chemical Technology in 2011. Now Su Zhang is pursuing his Ph.D. degree in Chemical Engineering and Technology in Beijing University of Chemical Technology under the guidance of Prof. Huaihe Song. His current research interest is on low dimensional carbon materials and energy storage devices.
Lingxiang Zhu received his bachelor degree in Materials Science and Engineering from Beijing University of Chemical Technology in 2012, and then worked as Research Assistant in Dr. Huaihe Song's group until 2013. Now Lingxiang is pursuing his Ph.D. degree in Chemical Engineering in University at Buffalo, The State University of New York. His current research interest is on graphene based materials as well as membranes for CO2 capture.
180 Huaihe Song obtained his Ph.D. degree in Chemical Processing from the Institute of Coal Chemistry, Chinese Academy of Sciences, China, in 1997. After two years of Postdoc Research in Beijing University of Chemical Technology (BUCT), he then became a Professor in College of Materials Science and Engineering, BUCT, from 2001. His main research interests are in the fields of carbon and carbon-based composite materials, including pitch-based carbon materials, carbon nanomaterials, ordered mesoporous carbons and carbon materials for energy storage, such as in lithium-ion batteries and supercapacitors. He has published more than 220 peer-reviewed papers up to now. Xiaohong Chen received her Bachelor degree in Taiyuan University of Technology in 1992 and Ph.D. degree from Beijing University of Chemical Technology in 1998. After two years of Postdoc Research in Max Planck Institute for Solid State Research, she joined Beijing University of Chemical Technology as a Professor in 2005. Her current research interests are mainly focusing on carbon nanomaterials, aerogel and bamboo charcoal. She has published more than 50 articles up to now.
S. Zhang et al. Jisheng Zhou is currently a lecturer in the Materials Science and Engineering College, Beijing University of Chemical Technology, China. He received his B.E. in Chemical Engineering from China University of Petroleum in 2005, and Ph.D. in Materials Science and Engineering from Beijing University of Chemical Technology in 2011. His research interests focuses on synthesis of nanocarbons and nanocarbon-based composites, and their applications in energy storage.